Chosen Ciphertext Security

Short recap

Let’s start by reviewing what we have learned so far:

We can mathematically define security for encryption schemes. A
natural definition is perfect secrecy: no matter what Eve does,
she can’t learn anything about the plaintext that she didn’t know
before. Unfortunately this requires the key to be as long as the
message, thus placing a severe limitation on the usability of it.

To get around this, we need to consider computational
considerations. A basic object is a pseudorandom generator and we
considered the PRG Conjecture which stipulates the existence of an
efficiently computable function
\(G:\{0,1\}^n\rightarrow\{0,1\}^{n+1}\) such that
\(G(U_n)\approx U_{n+1}\) (where \(U_m\) denotes the uniform
distribution on \(\{0,1\}^m\) and \(\approx\) denotes computational
indistinguishability).

We showed that the PRG conjecture implies a pseudorandom generator
of any polynomial output length which in particular via the stream
cipher construction implies a computationally secure encryption with
plaintext arbitrarily larger than the key. (The only restriction is
that the plaintext is of polynomial size which is anyway needed if
we want to actually be able to read and write it.)

We then showed that the PRG conjecture actually implies a stronger
object known as a pseudorandom function (PRF) function collection:
this is a collection \(\{ f_s \}\) of functions such that if we choose
\(s\) at random and fix it, and give an adversary a black box
computing \(i \mapsto f_s(i)\) then she can’t tell the difference
between this and a blackbox computing a random function.

Pseudorandom functions turn out to be useful for identification
protocols, message authentication codes and this strong notion of
security of encryption known as chosen plaintext attack (CPA)
security where we allow to encrypt many messages of Eve’s choice
and still require that the next message hides all information except
for what Eve already knew before.

Going beyond CPA

It may seem that we have finally nailed down the security definition for
encryption. After all, what could be stronger than allowing Eve
unfettered access to the encryption function. Clearly an encryption
satisfying this property will hide the contents of the message in all
practical circumstances. Or will it?

Please stop and play an ominous sound track at this point.

Example: The Wired Equivalence Protocol (WEP)

The WEP is perhaps one of the most inaccurately named protocols of all
times. It was invented in 1999 for the purpose of securing Wi-Fi
networks so that they would have virtually the same level of security as
wired networks, but already early on several security flaws were pointed
out. In particular in 2001, Fluhrer, Mantin, and Shamir showed how the
RC4 flaws we mentioned in prior lecture can be used to completely break
WEP in less than one minute. Yet, the protocol lingered on and for many
years after was still the most widely used WiFi encryption protocol as
many routers had it as the default option. In 2007 the WEP was blamed
for a hack stealing 45 million credit card numbers from T.J. Maxx. In
2012 (after 11 years of attacks!) it was estimated that it is still used
in about a quarter of encrypted wireless networks, and in 2014 it was
still the default option on many Verizon home routers. (I don’t know of
more recent surveys.) Here we will talk about a different flaw of WEP
that is in fact shared by many other protocols, including the first
versions of the secure socket layer (SSL) protocol that is used to
protect all encrypted web traffic.

To avoid superfluous details we will consider a highly abstract (and
somewhat inaccurate) version of WEP that still demonstrates our main
point. In this protocol Alice (the user) sends to Bob (the access point)
an IP packet that she wants routed somewhere on the internet.

Thus we can think of the message Alice sends to Bob as a string
\(m\in\{0,1\}^\ell\) of the form \(m=(m_1,m_2)\) where \(m_1\) is the IP
address this packet needs to be routed to and \(m_2\) is the actual
message that needs to be delivered. In the WEP protocol, the message
that Alice sends to Bob has the form\(E_k(m\|CRC(m))\) (where \(\|\) denotes concatenation and \(CRC(m)\) is some
cyclic redundancy code). The actual encryption WEP used was RC4, but for
us it doesn’t really matter. What does matter is that the encryption has
the form \(E_k(m') = pad \oplus m'\) where \(pad\) is computed as some
function of the key. In particular the attack we will describe works
even if we use our stronger CPA secure PRF-based scheme where
\(pad=f_k(r)\) for some random (or counter) \(r\) that is sent out
separately.

Now the security of the encryption means that an adversary seeing the
ciphertext \(c=E_k(m\|crc(m))\) will not be able to know \(m\), but since
this is traveling over the air, the adversary could “spoof” the signal
and send a different ciphertext \(c'\) to Bob. In particular, if the
adversary knows the IP address \(m_1\) that Alice was using (e.g., for
example, the adversary can guess that Alice is probably one of the
billions of people that visit the website boazbarak.org on a regular
basis) then she can XOR the ciphertext with a string of her choosing and
hence convert the ciphertext \(c = pad \oplus (m_1,m_2,CRC(m_1,m_2))\)
into the ciphertext \(c' = c \oplus x\) where \(x=(x_1,x_2,x_3)\) is
computed so that \(x_1 \oplus m_1\) is equal to the adversary’s own IP
address!

So, the adversary doesn’t need to decrypt the message- by spoofing the
ciphertext she can ensure that Bob (who is an access point, and whose
job is to decrypt and then deliver packets) simply delivers it
unencrypted straight into her hands. One issue is that Eve modifies
\(m_1\) then it is unlikely that the CRC code will still check out, and
hence Bob would reject the packet. However, CRC
32 - the CRC algorithm used by WEP - is linear
modulo \(2\), which means that for every pair of strings \(x_1,x_2\),
\(CRC(m_1\oplus x_1,m_2 \oplus m_2)=CRC(m_1,m_2)\oplus CRC(x_1,x_2)\).
This means that if the original ciphertext \(c\) was an encryption of the
message \(m=(m_1,m_2,CRC(m_1,m_2))\) then \(c'=c \oplus (x_1,0,CRC(x_1,0))\)
will be an encryption of the message
\(m'=(m_1 \oplus x_1, m_2, CRC(x_1\oplus m_1,m_2))\). (Where \(0\) denotes a
string of zeroes of the same length as \(m_2\), and hence
\(m_2 \oplus 0 = m_2\).) Therefore by XOR’ing \(c\) with
\((x_1,0,CRC(x_1,0))\), the adversary Mallory can ensure that Bob will
deliver the message \(m_2\) to the IP address \(m_1 \oplus x_1\) of her
choice (see Reference:WEPattackfig).

⊕The attack on the WEP protocol allowing the adversary Mallory to read
encrypted messages even when Alice uses a CPA secure
encryption.

Chosen ciphertext security

This is not an isolated example but in fact an instance of a general
pattern of many breaks in practical protocols. Some examples of
protocols broken through similar means include XML
encryption
, IPSec (see also
here) as well as JavaServer Faces,
Ruby on Rails, ASP.NET, and the Steam gaming client (see the Wikipedia
page on Padding Oracle Attacks).

The point is that often our adversaries can be active and modify the
communication between sender and receiver, which in effect gives them
access not just to choose plaintexts of their choice to encrypt but
even to have some impact on the ciphertexts that are decrypted. This
motivates the following notion of security (see also
Reference:CCAgamefig):

An encryption scheme \((E,D)\) is chosen ciphertext attack (CCA) secure
if every efficient adversary Mallory wins in the following game with
probability at most \(1/2+ negl(n)\):* Mallory gets \(1^n\) where \(n\) is the length of the key* For \(poly(n)\) rounds, Mallory gets access to the functions
\(m \mapsto E_k(m)\) and \(c \mapsto D_k(c)\).* Mallory chooses a pair of messages \(\{ m_0,m_1 \}\), a secret \(b\) is
chosen at random in \(\{0,1\}\), and Mallory gets \(c^* = E_k(m_b)\).* Mallory now gets another \(poly(n)\) rounds of access to the functions
\(m \mapsto E_k(m)\) and \(c \mapsto D_k(c)\) except that she is not allowed
to query \(c^*\) to her second oracle.* Mallory outputs \(b'\) and wins if \(b'=b\).

⊕the CCA security game

This might seems a rather strange definition so let’s try to digest it
slowly. Most people, once they understand what the definition says,
don’t like it that much. There are two natural objections to it:

This definition seems to be too strong: There is no way we would
let Mallory play with a decryption box - that basically amounts to
letting her break the encryption scheme. Sure, she could have some
impact on the ciphertexts that Bob decrypts and observe some
resulting side effects, but there is a long way from that to giving
her oracle access to the decryption algorithm.

The response to this is that it is very hard to model what is the
“realistic” information Mallory might get about the ciphertexts she
might cause Bob to decrypt. The goal of a security definition is not to
capture exactly the attack scenarios that occur in real life but rather
to be sufficiently conservative so that these real life attacks could
be modeled in our game. Therefore, having a too strong definition is not
a bad thing (as long as it can be achieved!). The WEP example shows that
the definition does capture a practical issue in security and similar
attacks on practical protocols have been shown time and again (see for
example the discussion of “padding attacks” in Section 3.7.2 of the Katz
Lindell book.)

This definition seems to be too weak: What justification do we
have for not allowing Mallory to make the query \(c^*\) to the
decryption box? After all she is an adversary so she could do
whatever she wants. The answer is that the definition would be
clearly impossible to achieve if Mallory could simply get the
decryption of \(c^*\) and learn whether it was an encryption of \(m_0\)
or \(m_1\). So this restriction is the absolutely minimal one we could
make without causing the notion to be obviously impossible. Perhaps
surprisingly, it turns out that once we make this minimal
restriction, we can in fact construct CCA-secure encryptions.

What does CCA has to do with WEP? The CCA security game is somewhat
strange, and it might not be immediately clear whether it has anything
to do with the attack we described on the WEP protocol. However, it
turns out that using a CCA secure encryption would have prevented that
attack. The key is the following claim:

Suppose that \((E,D)\) is a CCA secure encryption, then there is no
efficient algorithm that given an encryption \(c\) of the plaintext
\((m_1,m_2)\) outputs a ciphertext \(c'\) that decrypts to \((m'_1,m_2)\)
where \(m'_1\neq m_1\).

In particular Reference:ccaweplem rules out the attack of transforming
\(c\) that encrypts a message starting with a some address \(IP\) to a
ciphertext that starts with a different address \(IP'\). Let us now sketch
its proof.

We’ll show that such if we had an adversary \(M'\) that violates the
conclusion of the claim, then there is an adversary \(M\) that can win in
the CCA game.

The proof is simple and relies on the crucial fact that the CCA game
allows \(M\) to query the decryption box on any ciphertext of her
choice, as long as it’s not exactly identical to the challenge
cipertext \(c^*\). In particular, if \(M'\) is able to morph an encryption
\(c\) of \(m\) to some encryption \(c'\) of some different \(m'\) that agrees
with \(m\) on some set of bits, then \(M\) can do the following: in the
security game, use \(m_0\) to be some random message and \(m_1\) to be this
plaintext \(m\). Then, when receiving \(c^*\), apply \(M'\) to it to obtain a
ciphertext \(c'\) (note that if the plaintext differs then the ciphertext
must differ also; can you see why?) ask the decryption box to decrypt it
and output \(1\) if the resulting message agrees with \(m\) in the
corresponding set of bits (otherwise output a random bit). If \(M'\) was
successful with probability \(\epsilon\), then \(M\) would win in the CCA
game with probability at least \(1/2 + \epsilon/10\) or so.

The proof above is rather sketchy. However it is not very difficult and
proving Reference:ccaweplem on your own is an excellent way to ensure
familiarity with the definition of CCA security.

Constructing CCA secure encryption

The definition of CCA seems extremely strong, so perhaps it is not
surprising that it is useful, but can we actually construct it? The WEP
attack shows that the CPA secure encryption we saw before (i.e.,
\(E_k(m)=(r,f_k(r)\oplus m)\)) is not CCA secure. We will see other
examples of non CCA secure encryptions in the exercises. So, how do
we construct such a scheme? The WEP attack actually already hints of the
crux of CCA security. We want to ensure that Mallory is not able to
modify the challenge ciphertext \(c^*\) to some related \(c'\). Another way
to say it is that we need to ensure the integrity of messages to
achieve their confidentiality if we want to handle active
adversaries that might modify messages on the channel. Since in in a
great many practical scenarios, an adversary might be able to do so,
this is an important message that deserves to be repeated:

To ensure confidentiality, you need integrity.

This is a lesson that has been time and again been shown and many
protocols have been broken due to the mistaken belief that if we only
care about secrecy, it is enough to use only encryption (and one
that is only CPA secure) and there is no need for authentication.
Matthew
Green
writes this more provocatively as

Nearly all of the symmetric encryption modes you learned about in
school, textbooks, and Wikipedia are (potentially) insecure.I also like the part where Green says about a block cipher mode
that “if OCB was your kid, he’d play three sports and be on his way
to Harvard.” We will have an exercise about a simplified version of
the GCM mode (which perhaps only plays a single sport and is on its
way to …). You can read about OCB in Exercise 9.14 in the
Boneh-Shoup book; it uses the notion of a “tweakable block cipher”
which simply means that given a single key \(k\), you actually get a
set \(\{ p_{k,1},\ldots,p_{k,t} \}\) of permutations that are
indistinguishable from \(t\) independent random permutation (the set
\(\{1,\ldots, t\}\) is called the set of “tweaks” and we sometimes
index it using strings instead of numbers).

exactly because these basic modes only ensure security for passive
eavesdropping adversaries and do not ensure chosen ciphertext security
which is the “gold standard” for online applications. (For symmetric
encryption people often use the name “authenticated encryption” in
practice rather than CCA security; those are not identical but are
extremely related notions.)

All of this suggests that Message Authentication Codes might help us get
CCA security. This turns out to be the case. But one needs to take some
care exactly how to use MAC’s to get CCA security. At this point, you
might want to stop and think how you would do this…

You should stop here and try to think how you would implement a CCA
secure encryption by combining MAC’s with a CPA secure encryption.

\newpage

If you didn’t stop before, then you should really stop and think now.

\newpage

OK, so now that you had a chance to think about this on your own, we
will describe one way that works to achieve CCA security from MACs. We
will explore other approaches that may or may not work in the exercises.

Let \((E,D)\) be CPA-secure encryption scheme and \((S,V)\) be a CMA-secure
MAC with \(n\) bit keys and a canonical verification algorithm.By a canonical verification algorithm we mean that
\(V_k(m,\sigma)=1\) iff \(S_k(m)=\sigma\). Then
the following encryption \((E',D')\) with keys \(2n\) bits is CCA secure:* \(E'_{k_1,k_2}(m)\) is obtained by computing \(c=E_{k_1}(m)\) ,
\(\sigma = S_{k_2}(c)\) and outputting \((c,\sigma)\).* \(D'_{k_1,k_2}(c,\sigma)\) outputs nothing (e.g., an error message) if
\(V_{k_2}(c,\sigma)\neq 1\), and otherwise outputs \(D_{k_1}(c)\).

Suppose, for the sake of contradiction, that there exists an adversary
\(M'\) that wins the CCA game for the scheme \((E',D')\) with probability at
least \(1/2+\epsilon\). We consider the following two cases:

Case I: With probability at least \(\epsilon/10\), at some point
during the CCA game, \(M'\) sends to its decryption box a ciphertext
\((c,\sigma)\) that is not identical to one of the ciphertexts it
previously obtained from its decryption box, and obtains from it a
non-error response.

We will derive a contradiction in either case. In the first case, we
will use \(M'\) to obtain an adversary that breaks the MAC \((S,V)\), while
in the second case, we will use \(M'\) to obtain an adversary that breaks
the CPA-security of \((E,D)\).

Let’s start with Case I: When this case holds, we will build an
adversary \(F\) (for “forger”) for the MAC \((S,V)\), we can assume the
adversary \(F\) has access to the both signing and verification algorithms
as black boxes for some unknown key \(k_2\) that is chosen at random and
fixed.Since we use a MAC with canonical verification, access to the
signature algorithm implies access to the verification algorithm. \(F\) will choose \(k_1\) on its own, and will also choose at
random a number \(i_0\) from \(1\) to \(T\), where \(T\) is the total number of
queries that \(M'\) makes to the decryption box. \(F\) will run the entire
CCA game with \(M'\), using \(k_1\) and its access to the black boxes to
execute the decryption and decryption boxes, all the way until just
before \(M'\) makes the \(i_0^{th}\) query \((c,\sigma)\) to its decryption
box. At that point, \(F\) will output \((c,\sigma)\). We claim that with
probability at least \(\epsilon/(10T)\), our forger will succeed in the
CMA game in the sense that (i) the query \((c,\sigma)\) will pass
verification, and (ii) the message \(c\) was not previously queried
before to the signing oracle.

Indeed, because we are in Case I, with probability \(\epsilon/10\), in
this game some query that \(M'\) makes will be one that was not asked
before and hence was not queried by \(F\) to its signing oracle, and
moreover, the returned message is not an error message, and hence the
signature passes verification. Since \(i_0\) is random, with probability
\(\epsilon/(10T)\) this query will be at the \(i_0^{th}\) round. Let us
assume that this above event \(GOOD\) happened in which the \(i_0\)-th query
to the decryption box is a pair \((c,\sigma)\) that both passes
verification and the pair \((c,\sigma)\) was not returned before by the
encryption oracle. Since we pass (canonical) verification, we know that
\(\sigma=S_{k_2}(c)\), and because all encryption queries return pairs of
the form \((c',S_{k_2}(\sigma'))\), this means that no such query returned
\(c\) as its first element either. In other words, when the event \(GOOD\)
happens the \(i_0\)-the query contains a pair \((c,\sigma)\) such that \(c\)
was not queried before to the signature box, but \((c,\sigma)\) passes
verification. This is the definitoin of breaking \((S,V)\) in a chosen
message attack, and hence we obtain a contradiction to the CMA security
of \((S,V)\).

Now for Case II: In this case, we will build an adversary \(Eve\) for
CPA-game in the original scheme \((E,D)\). As you might expect, the
adversary \(Eve\) will choose by herself the key \(k_2\) for the MAC scheme,
and attempt to play the CCA security game with \(M'\). When \(M'\) makes
encryption queries this should not be a problem- \(Eve\) can forward the
plaintext \(m\) to its encryption oracle to get \(c=E_{k_1}(m)\) and then
compute \(\sigma = S_{k_2}(c)\) since she knows the signing key \(k_2\).

However, what does \(Eve\) do when \(M'\) makes decryption queries? That
is, suppose that \(M'\) sends a query of the form \((c,\sigma)\) to its
decryption box. To simulate the algorithm \(D'\), \(Eve\) will need access
to a decryption box for \(D\), but she doesn’t get such a box in the CPA
game (This is a subtle point- please pause here and reflect on it until
you are sure you understand it!)

To handle this issue \(Eve\) will follow the common approach of “winging
it and hoping for the best”. When \(M'\) sends a query of the form
\((c,\sigma)\), \(Eve\) will first check if it happens to be the case that
\((c,\sigma)\) was returned before as an answer to an encryption query
\(m\). In this case \(Eve\) will breathe a sigh of relief and simply return
\(m\) to \(M'\) as the answer. (This is obviously correct: if \((c,\sigma)\)
is the encryption of \(m\) then \(m\) is the decryption of \((c,\sigma)\).)
However, if the query \((c,\sigma)\) has not been returned before as an
answer, then \(Eve\) is in a bit of a pickle. The way out of it is for her
to simply return “error” and hope that everything will work out. The
crucial observation is that because we are in case II things will work
out. After all, the only way \(Eve\) makes a mistake is if she returns an
error message where the original decryption box would not have done so,
but this happens with probability at most \(\epsilon/10\). Hence, if \(M'\)
has success \(1/2+\epsilon\) in the CCA game, then even if it’s the case
that \(M'\) always outputs the wrong answer when \(Eve\) makes this mistake,
we will still get success at least \(1/2+0.9\epsilon\). Since \(\epsilon\)
is non negligible, this would contradict the CPA security of \((E,D)\)
therby concluding the proof of the theorem.

This proof is emblematic of a general principle for proving CCA
security. The idea is to show that the decryption box is completely
“useless” for the adversary, since the only way to get a non error
response from it is to feed it with a ciphertext that was received from
the encryption box.

(Simplified) GCM encryption

The construction above works as a generic construction, but it is
somewhat costly in the sense that we need to evaluate both the block
cipher and the MAC. In particular, if messages have \(t\) blocks, then we
would need to invoke two cryptographic operations (a block cipher
encryption and a MAC computation) per block. The GCM (Galois Counter
Mode) is a way around this. We are going to
describe a simplified version of this mode. For simplicity, assume that
the number of blocks \(t\) is fixed and known (though many of the annoying
but important details in block cipher modes of operations involve
dealing with padding to multiple of blocks and dealing with variable
block size).

A universal hash function collection is a
family of functions \(\{ h:\{0,1\}^\ell\rightarrow\{0,1\}^n \}\) such that
for every \(x \neq x' \in \{0,1\}^\ell\), the random variables \(h(x)\) and
\(h(x')\) (taken over the choice of a random \(h\) from this family) are
pairwise independent in \(\{0,1\}^{2n}\). That is, for every two potential
outputs \(y,y'\in \{0,1\}^n\), \[
\Pr_h[ h(x)=y \;\wedge\; h(x')=y']=2^{-2n} \label{equnivhash}
\]

Universal hash functions have rather efficient constructions, and in
particular if we relax the definition to allow almost universal hash
functions (where we replace the \(2^{-2n}\) factor in the righthand side
of \eqref{equnivhash} by a slightly bigger, though still negligible
quantity) then the constructions become extremely efficient and the size
of the description of \(h\) is only related to \(n\), no matter how big
\(\ell\) is.In \(\epsilon\)-almost universal hash functions we require that for
every \(y,y'\in \{0,1\}^{n}\), and \(x\neq x' \in \{0,1\}^\ell\), the
probability that \(h(x)= h(x')\) is at most \(\epsilon\). It can be
easily shown that the analysis below extends to \(\epsilon\) almost
universal hash functions as long as \(\epsilon\) is negligible, but we
will leave verifying this to the reader.

Our encryption scheme is defined as follow. The key is \((k,h)\) where \(k\)
is an index to a pseudorandom permutation \(\{ p_k \}\) and \(h\) is the key
for a universal hash function.In practice the key \(h\) is derived from the key \(k\) by applying
the PRP to some particular input. To encrypt a message
\(m = (m_1,\ldots,m_t) \in \{0,1\}^{nt}\) do the following:

Choose \(IV\) at random in \([2^n]\).

Let \(z_i = E(k,IV+i)\) for \(i=1,\ldots,t+1\).

Let \(c_i = z_i \oplus m_i\).

Let \(c_{t+1} = h(c_1,\ldots,c_t) \oplus z_{t+1}\).

Output \((IV,c_1,\ldots,c_{t+1})\).

The communication overhead includes one additional output block plus the
IV (whose transmission can often be avoided or reduced, depending on the
settings; see the notion of “nonce based encryption”). This is fairly
minimal. The additional computational cost on top of \(t\) block-cipher
evaluation is the application of \(h(\cdot)\). For the particular choice
of \(h\) used in Galois Counter Mode, this function \(h\) can be evaluated
very efficiently- at a cost of a single multiplication in the Galois
field of size \(2^{128}\) per block (one can think of it as some very
particular operation that maps two \(128\) bit strings to a single one,
and can be carried out quite efficiently). We leave it as an
(excellent!) exercise to prove that the resulting scheme is CCA secure.

Padding, chopping and their pitfalls: the “buffer overflow” of cryptography

In this course we typically focus on the simplest case where messages
have a fixed size. But in fact, in real life we often need to chop
long messages into blocks, or pad messages so that their length becomes
an integral multiple of the block size. Moreover, there are several
subtle ways to get this wrong, and these have been used in several
practical attacks.

Chopping into blocks: A block cipher a-priori provides a way to
encrypt a message of length \(n\), but we often have much longer messages
and need to “chop” them into blocks. This is where the block cipher
modes discussed in the previous lecture come in. However, the basic
popular modes such as CBC and OFB do not provide security against
chosen ciphertext attack, and in fact typically make it easy to extend
a ciphertext with an additional block or to remove the last block from
a ciphertext, both being operations which should not be feasible in a
CCA secure encryption.

Padding: Oftentimes messages are not an integer multiple of the
block size and hence need to be padded. The padding is typically a
map that takes the last partial block of the message (i.e., a string \(m\)
of length in \(\{0,\ldots,n-1\}\)) and maps it into a full block (i.e., a
string \(m\in\{0,1\}^n\)). The map needs to be invertible which in
particular means that if the message is already an integer multiple of
the block size we will need to add an extra block. (Since we have to map
all the \(1+2+\ldots+2^{n-1}\) messages of length \(1,\ldots,n-1\) into the
\(2^n\) messages of length \(n\) in a one-to-one fashion.) One approach for
doing so is to pad an \(n'<n\) length message with the string
\(10^{n-n'-1}\). Sometimes people use a different padding which involves
encoding the length of the pad.

Chosen ciphertext attack as implementing metaphors

The classical “metaphor” for an encryption is a sealed envelope, but as
we have seen in the WEP, this metaphor can lead you astray. If you
placed a message \(m\) in a sealed envelope, you should not be able to
modify it to the message \(m \oplus m'\) without opening the envelope, and
yet this is exactly what happens in the canonical CPA secure encryption
\(E_k(m)=(r,f_k(r) \oplus m)\). CCA security comes much closer to
realizing the metaphor, and hence is considered as the “gold standard”
of secure encryption. This is important even if you do not intend to
write poetry about encryption. Formal verification of computer
programs is an area that is growing in importance given that computer
programs become both more complex and more mission critical.
Cryptographic protocols can fail in subtle ways, and even published
proofs of security can turn out to have bugs in them. Hence there is a
line of research dedicated to finding ways to automatically prove
security of cryptographic protocols. Much of these line of research is
based on simple models to describe protocols that are known as Dolev
Yao models, based on the first paper that proposed such models. These
models define an algebraic form of security, where rather than
thinking of messages, keys, and ciphertexts as binary string, we think
of them as abstract entities. There are certain rules for manipulating
these symbols. For example, given a key \(k\) and a message \(m\) you can
create the ciphertext \(\{ m \}_k\), which you can decrypt back to \(m\)
using the same key. However the assumption is that any information that
cannot be obtained by such manipulation is unknown.

Translating a proof of security in this algebra to proof for real world
adversaries is highly non trivial. However, to have even a fighting a
chance, the encryption scheme needs to be as strong as possible, and in
particular it turns out that security notions such as CCA play a crucial
role.